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Shovelhead ignition problem - missing under load
hello,
Desperate to keep my buddies 81 FLHT out of the dealership to pay for troubleshooting time, we've decided to figure this one out on our own. "Betty" is an 81 shovel with a S&S Super B(i'm pretty sure, don't believe it has an accelerator pump,) and a Crane Hi-4 ignition Dual fire ignition unit. It was rebuilt 4 years and 12,000 miles ago when it was purchased. Not sure about cams or internals. The compression tests ran earlier in the month came back good and it just had a new rotor installed (spun the old one.) After getting it back form the local wrench it ran excellent for 50 miles. We stopped, got some lunch and the following issue started an hour later.
The issue is this, when under any type of load the bike starts missing and backfiring, and makes no power. It will idle fine and sometimes revs in neutral fine. Other times when reving without load, the bike will miss at about 2500 RPM. But, under load she fails completely with horrible backfiring and lots of unburnt fuel smell. We were worried the Crane Hi-4 was dying so we ran the prescribed test of idling the bike and heating up the ignition module with a blow dryer. She never died. Actually, when the issue first started the bike would always start back up, but under load she would start missing and failing. We have checked for bad gas, clogged fuel filter without any success.
I am wondering if a worn out coil could cause these types of problems? I have wrenched on cars a little, but never on a shovel, so any help in diagnosis would be great.
Thanks,
Boomer
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